1. Field of the Invention
The present invention relates to a low-density, high-strength, gas-permeable, porous silicon carbide sinter, having a three-dimensional network structure comprising plate crystals of silicon carbide, and its production process. More particularly the present invention relates to a heat-resistant, corrosion-resistant, porous silicon carbide sinter which can be used suitably as a catalyst or catalyst support for chemical reactions, a material for a filter used in removing fine-particulate substances contained in high-temperature gases, or a material for heat exchangers, and its production process.
Silicon carbide has excellent chemical and physical properties such as excellent hardness, excellent abrasion resistance, excellent oxidation resistance, excellent corrosion resistance, good heat conductivity, low coefficient of thermal expansion, high thermal-shock resistance and high-temperature strength, and therefore it is a material which can be widely used as an abrasion-resistant material for mechanical seals and bearings, refractories of high-temperature furnaces, heat-resistant structural materials for heat exchangers, combustion tubes, etc., and corrosion-resistant materials for parts of a pump for strongly corrosive solutions of acids, alkalis, etc.
On the other hand, it is known that, by virtue of the above-mentioned properties of silicon carbide, a porous silicon carbide sinter comprising silicon carbide and gas-permeable pores, that is, open pores (hereinafter, referred to simply as pores) formed by crystals of silicon carbide can be used as a material which can be utilized in applications where the above-mentioned properties of silicon carbide are fully utilized, such as filters used in a high-temperature atmosphere, an oxidizing atmosphere, and/or a corrosive atmosphere, or a catalyst or catalyst support for exothermic oxidation reactions or chemical reactions at high-temperatures, and, for example, it is used as a filter for removing combustible fine-particulate substances, such as fine-particulate carbon contained in high-temperature gases such as exhaust gas from an internal combustion engine, especially, exhaust gas from a Diesel engine.
When the porous silicon carbide sinter is used as a filter such as the above-mentioned, it must have not merely heat resistance and corrosion resistance but also properties such as low permeation resistance to a fluid, an ability to remove foreign particles in high efficiency and a long period of endurance. On the other hand, when the porous silicon carbide sinter is used as a catalyst or a catalyst support or parts of a heat exchanger, it must satisfy requirements such as a high surface area for performing a chemical reaction, or a heat or material transfer effectively, and besides longterm stability of the surface.
2. Description of the Prior Art
Conventional processes for producing a porous silicon carbide sinter include (1) one comprising adding a binder such as a glass flux or clay to silicon carbide particles as an aggregate, molding the mixture and sintering the molding at a temperature at which the binder can melt, (2) one comprising mixing coarser silicon carbide particles with finer silicon carbide particles, molding the mixture, and sintering the molding at a temperature as high as above 2000.degree. C., and (3) one disclosed in Japanese Patent Laid-Open No. 39515/1973, that is, "a process for producing a uniform porous recrystallized silicon carbide body, comprising adding a carbonaceous binder with or without carbon powder to silicon carbide powder, adding silicaceous powder in a theoretical amount necessary to react with the added carbon and free carbon formed from the binder during baking, molding the mixture, and thereafter heating the molding to 1,900.degree. to 2,400.degree. C. in a carbon powder to silicify the carbon in the molding".
The porous body produced by adding a glass flux or clay which serves as a binder in process (1) has a drawback that, because the binder melts at 1,000.degree. to 1,400.degree. C., the porous body is deformed in this temperature range, especially, near the glass transition point, and decreases markedly in its strength, with consequent limited applicability to fields which require both chemical and oxidation resistances.
On the other hand, the structure of the porous body produced by the above process (2) or (3), which is shown schematically in FIG. 3, is composed of a silicon carbide aggregate (A), a silicon carbide binder or a carbonaceous binder (B) which binds the aggregate particles together by coating and voids (C). The distribution of the voids, i.e., pores of the above porous body is determined substantially by the arrangement of aggregate particles during molding, and the porosity of the sinter is as low as 30 to 40%. Therefore, the permeation resistance of the porous body to a fluid is markedly high. On the other hand, when the porosity of the sinter is increased, there is a tendency that the number of contact points among aggregate particles is decreased, the strength of the porous body is decreased markedly, and the area of contact with a fluid is decreased markedly.
On the other hand, according to the process (2) or (3), the control of the pore diameter in the porous body is performed by blending aggregates of different particle sizes. According to these processes, in order to obtain a porous body having pores of a relatively large sectional area, an aggregate of a larger particle size is necessary and therefore the number of contact points among particles is reduced and the bonding strength of the particles is decreased, with a consequent decrease in the strength of the porous body. On the other hand, in order to obtain a porous body having pores of a relatively small sectional area, it is necessary to mold a mixture obtained by suitably blending coarser aggregate particles with medium aggregate particles and/or finer aggregate particles, and therefore there is a tendency that the porosity of the molding is decreased markedly and, in an extreme case, the pores are blocked. Therefore, the permeation resistance of such a porous body to a fluid is markedly high.
Further, with respect to porous sinters having pores of a relatively larger sectional area, Japanese Patent Laid-Open No. 122016/1983, for example, discloses a process for producing an electrically heat-generative silicon carbide filter, comprising impregnating a polymer foam material with a silicon carbide-based slurry, eliminating said polymer foam material by heat treatment to form a silicon carbide-based skeletal structure, subjecting said structure to the primary baking in argon gas at a temperature of 1,900.degree. to 2,300.degree. C., subjecting it to the secondary baking in nitrogen gas at a pressure of 1 to 200 atm and a temperature of 1,600.degree. to 2,100.degree. C., and forming a heat-resistant electrode on each end of the structure to make it possible to pass an electric current therethrough, and Japanese Patent Laid-Open No. 81905/1973 discloses a process for producing a porous ceramic material, comprising impregnating an organic foam with a slurry containing a finely divided organic material, drying the foam thus impregnated, and baking the dried product, wherein the foam is impregnated with the slurry, after it is treated so that the particulate material in the slurry may adhere to the surface of the foam structure.
Such porous bodies are composed of a cellular skeletons of various sizes, i.e., so-called skeletal structure, as shown in FIG. 4. Therefore, when the porous body is occupied by relatively large cellular skeleton (D), the porosity of the body is as high as 80 to 90% by volume, and permeation resistance is decreased but the strength is 10 to 15 kg/cm.sup.2. Therefore, from the viewpoint of its practicality, the porous body has a drawback that it is relatively poor in mechanical strength and its area of contact with a fluid is markedly small. Further, according to these production processes, the pores which the polymer foam such as polyurethane constitutes tend to have a size of 100 .mu.m or larger, and the formation of pores below 100 .mu.m is very difficult in respect of the control of the expandability and dispersibility of a polymer and, in some cases, part of the open pores are turned into closed ones, or the diameters of open pores formed in the cell walls are small relative to the inside void volume, so that there is a drawback that the permeation resistance is too large to pass a fluid therethrough.